The present invention relates generally to filtration membranes and to methods for preparing filtration membranes and relates more particularly to a novel method for preparing a filtration membrane and to a filtration membrane prepared by said method.
Filtration is a mechanical process used to separate solids from fluids using a porous medium, i.e., a filtration membrane, through which only the fluid and those particles smaller than the pores of the filtration membrane can pass. Consequently, depending on the pore size of filtration membrane, one can filter solids of a corresponding size. Various classes of filtration membranes exist, such classes including microfiltration membranes, ultrafiltration membranes, and reverse osmosis membranes. Microfiltration membranes are typically capable of filtering solids larger than about 0.05 microns, ultrafiltration membranes are typically capable of filtering solids larger than about 0.002 microns, and reverse osmosis membranes are typically capable of filtering solids larger than about 0.0006 microns.
Microfiltration membranes and ultrafiltration membranes are typically made by the same type of phase inversion process of a polymer solution, with either a microfiltration membrane or an ultrafiltration membrane being produced depending upon the type of polymer in the solution, the concentration of polymer in the solution, and the type of solvent in the solution. As seen, for example, in U.S. Pat. No. 3,988,245, inventor Wang, which issued Oct. 26, 1976; U.S. Pat. No. 4,629,563, inventor Wrasidlo, which issued Dec. 16, 1986; and U.S. Pat. No. 5,886,059, inventor Wang, which issued Mar. 23, 1999, all of which are incorporated herein by reference, the membrane preparation process typically comprises casting a polymer solution onto a support, quenching the coated support in a water bath to dissolve out the initial solvent and to form the porous membrane, and then drying the formed membrane.
Typically, membranes of the type described above are formed as part of a continuous manufacturing process in which, after the membrane material is dried, the membrane is wound into a roll for subsequent processing into various end-use structures, such as pleated cartridges, spiral-wound membranes, and plate-and-frame membranes. According to one approach, the support onto which the polymer solution is cast is made of a non-porous polymeric material, such as a polyethylene terephthalate (PET) film, or other non-porous materials like glass. One benefit to using a non-porous material as the support is that, after the formation of the membrane on the support, the support separates from the membrane, thereby facilitating the subsequent processing of the membrane into the various types of end-use structures described above. However, on the other hand, because the membrane, unaccompanied by the support, possesses poor tensile strength, the manufacturing process must be conducted slowly in order to permit the membrane to be wound into a roll without being torn. Consequently, the throughput for manufacturing ultrafiltration and microfiltration membranes using a non-porous material as the support tends to be lower than desired.
In view of the above, another approach to manufacturing ultrafiltration and microfiltration membranes has been to use a support made of a woven or non-woven fabric, typically made of PET fibers, instead of a non-porous support made of a polymeric film or the like. When the polymer solution is cast onto the aforementioned fabric, the solution tends to penetrates into the fabric to a certain extent, thereby resulting in a membrane that does not, thereafter, separate from the support. One benefit to the support remaining coupled to the membrane is that the winding of the membrane (with its attached support) into a roll can be performed under higher tension and at greater speeds than would be possible if the membrane were unaccompanied by the support. On the other hand, because the support remains coupled to the membrane, the thickness of the support/membrane composite is greater than the thickness of the membrane alone, and this increased thickness limits the ability of the membrane to be processed into end-use structures, such as pleated cartridges, having optimally high surface areas.
As noted above, in addition to microfiltration membranes and ultrafiltration membranes, reverse osmosis membranes represent another class of filtration membranes. One common type of reverse osmosis membrane is a composite membrane comprising a porous support and a thin polyamide film formed on the porous support. Typically, the porous support is an ultrafiltration membrane formed on PET fabric, and the thin polyamide film is formed by an interfacial polymerization of a polyfunctional amine and a polyfunctional acyl halide. Examples of composite polyamide reverse osmosis membranes are disclosed in the following patents, all of which are incorporated herein by reference: U.S. Pat. No. 4,277,344, inventor Cadotte, which issued Jul. 7, 1981; U.S. Pat. No. 4,872,984, inventor Tomaschke, which issued Oct. 10, 1989; U.S. Pat. No. 4,983,291, inventors Chau et al., which issued Jan. 8, 1991; U.S. Pat. No. 5,576,057, inventors Hirose et al., which issued Nov. 19, 1996; U.S. Pat. No. 5,614,099, inventors Hirose et al., which issued Mar. 25, 1997; U.S. Pat. No. 4,950,404, inventor Chau, which issued Aug. 21, 1990; U.S. Pat. No. 4,830,885, inventors Tran et al., which issued May 16, 1989; U.S. Pat. No. 6,245,234, inventors Koo et al., which issued Jun. 12, 2001; U.S. Pat. No. 6,063,278, inventors Koo et al., which issued May 16, 2000; and U.S. Pat. No. 6,015,495, inventors Koo et al., which issued Jan. 18, 2000.
As can be appreciated, because composite polyamide membranes of the type described above typically include, in addition to a thin polyamide layer, a fabric support secured to an ultrafiltration membrane, the types of shortcomings discussed above that result from the use of a fabric support are also applicable to these composite polyamide membranes.
Another use for composite polyamide membranes of the aforementioned type is in a process known as forward osmosis. Forward osmosis is a natural phenomenon in which water flows through a porous membrane from a volume of low solute concentration to a volume of high solute concentration. As such, forward osmosis is effectively the opposite of reverse osmosis, in which a volume of high solute concentration is placed under sufficient pressure to exceed the osmotic pressure and, thereby, to cause water to flow from a volume of high solute concentration to a volume of low solute concentration. One application of the principle of forward osmosis has been used in a process called pressure retarded osmosis, which, as explained in the following documents incorporated herein by reference, has been used to produce electricity from the osmotic power of sea water: Loeb, “Large-scale power production by pressure-retarded osmosis, using river water and sea water passing through spiral modules,” Desalination, 143:115-22 (2002); McCutcheon et al., “A novel ammonia-carbon dioxide forward (direct) osmosis desalination process,” Desalination, 174:1-11 (2005); and Cath et al., “Forward osmosis: Principles, applications, and recent developments,” Journal of Membrane Science, 281:70-87 (2006).
In both reverse osmosis and forward osmosis, the thin polyamide layer faces the volume of high solute concentration, and the fabric support faces the volume of low solute concentration. In the case of reverse osmosis, whatever resistance the fabric support may provide to the flow of water from the volume of high solute concentration to the volume of low solute concentration is adequately overcome by the applied pressure. By contrast, in the case of forward osmosis, which relies solely on osmotic pressure to cause water to flow from the volume of low solute concentration to the volume of high solute concentration, the fabric support provides an undesirable resistance to water flow.